The heating of water, e.g. for generating steam, may be performed in water heating apparatuses or boilers. In these systems, the temperature of the water can be controlled within a certain temperature range by means of a heating device and a temperature sensor as follows: When the temperature signal of the temperature sensor indicates, that the temperature of the water falls below a certain level, the heating device is activated and the water is heated. If the temperature signal indicates, that the water temperature rises above a certain level, the heating device is deactivated.
Heating the water for the generation of steam requires water heating means under pressure and a control of the pressure of the steam. The controlling of the steam pressure can be performed directly by the use of a pressure sensor or indirectly by the use of a temperature sensor. Controlling the pressure by sensing the water temperature makes use of the correlation of the steam pressure and the temperature in the boiler, since during a heating of the water the steam pressure rises, and it decreases, when the water in the boiler is cooling down.
For controlling the pressure in the boiler on the basis of the measured temperature, the temperature of the water needs to be sensed accurately. In particular, the arrangement of the temperature sensor is critical. The sensor may be attached to the side walls of the boiler shell or to the bottom of the boiler shell.
Arranging the temperature sensor at the side walls requires a flat portion for a proper mounting of the sensor, which in turn complicates the forming of the shell. In some of these arrangements a heat conductive paste is applied between the temperature sensor and the boiler shell. This makes additional mounting processes necessary.
Attaching the temperature sensor at the bottom of the boiler shell also is disadvantageously. Some boilers comprise a heating plate with an embedded heating element. The heating plate usually is mounted to the bottom of the boiler shell by means of bolts or screws. A layer of thermal conducting material, e.g. graphite, may be arranged between the boiler and the heating plate to fill the air gap and to improve the heat transfer. However, the heat transfer between the boiler shell and the heating plate is not optimal. Especially during power up the water temperature and the temperature of the heating plate differ considerably. This causes a time delay in the temperature-time curve at the sensing location compared with the temperature-time curve of the water, since the heat transfer from the heating element into the water is considerably delayed. Furthermore, the spatial and temporal temperature distribution in the boiler is not even. For example, water within the sensing area of a sensor attached remotely from the heating device may be heated up later than water within the region of the heating device. This tends to cause either an overshooting of the steam pressure or the opposite.
It is an object of the invention to provide an apparatus and a method of generating steam providing an improved capability of controlling the steam pressure.